However, this Mailly, who fought in many battles of the wars of Louis XV, received from Louis XVI, in 1790, command of one of the four armies decreed by the National Assembly (14th and 15th military divisions).
This was a difficult task and he resigned on 22 June, when he learned of the king's flight to Varennes.
On 10 August 1792, despite his old age, he fought on the side of the threatened French monarchy.
Escaping the carnage that followed the capture of the palais des Tuileries and the September massacres, he was arrested in his château, then guillotined in 1794 at Arras, aged 87 – on the scaffold he cried "I remain faithful to my king, as my ancestors have always been".